Grinding mineral materials

ABSTRACT

A continuous output of very fine grain size mineral material is obtained in a mill grinding operation by initially grinding material to have a residue below 10 percent on a 210-micron screen, then further grinding the material by means of grinding bodies having a maximum middle or average weight of less than 15 grams until a product of a fineness below 1 percent on a 210micron screen is obtained and then the product is passed out of the mill by gravity or overflow through an opening at the outlet end of the mill.

United States Patent (72] lnventor Gunnar R. Fagerholt Copenhagen-Valby, Denmark [21] Appl. No. 688,859 [22] Filed Dec.7, 1967 {45] Patented June 22, 1971 [73] Assignee F. L. Smidth & Co.

New York City, N.Y.

[54] GRINDING MINERAL MATERIALS 2 Claims, 3 Drawing Figs.

{52] U.S.Cl .Q 241/30, 241/l76,241/l8l [51] Int. Cl B024: 17/06 [50] FieldofSearch .1 24l/2l.29, 15,14, 30, 26, 60

[56] References Cited UNITED STATES PATENTS 3,529,781 9/1970 Hastrup 241/181 1,368,739 2/1921 Lindhard 241/60 X 1,649,813 11/1927 Dalgaard 241/30 1,728,496 9/1929 Lindhard... 241/23 2,332,701 10/1943 Dowsett 241/29 2,394,052 2/1946. Hall 241/29 X I 2,922,585 1/1960 Searfoss 241/791 X Primary Examiner-Donald G. Kelly Anorney-Pennie, Edmonds, Morton, Taylor and Adams GRINDING MINERAL MATERIALS BACKGROUND or THE INVENTION This invention relates to improvements in methods of grinding mineral materials in a mill and to improvements in the mills for performing the method.

The grinding of mineral materials such as the raw materials used in the manufacture of cement is normally effected in a grinding mill, such as a ball or tube mill, which has a charge of loose grinding bodies.

In known mills in which coarse material is ground by means to a charge of loose grinding bodies it is common to divide the interior of the mill into two or more grinding chambers separated by perforated partitions, commonly known as diaphragms. Normally each chamber has a charge of grinding bodies, the size of which depends on the degree of fineness required in the ground product. The diaphragms serve essentially as screens in allowing the ground material to pass but in retaining the grinding bodies. At the exit of the last chamber of the mill, or at the exit of mills having only one chamber, there is usually a similar diaphragm to prevent the grinding bodies from leaving the mill. The ground material may leave the mill directly through a diaphragm that constitutes the end wall of a grinding chamber or, in the overflow type of mill, it may leave through a hollow trunnion having a diaphragm at its entrance.

In order to obtain a fine-grained product one of two methods is normally used. In one the grinding is effected in a mill with a number of grinding chambers as described above, each chamber having a charge of grinding bodies of a size decreasing in the direction of flow of the material.

In the other a relatively coarse material is first obtained in one step and this coarse material is then ground to a finegrained product in a mill in a second step. Neither method is wholly satisfactory, and in particular the output of the second method is limited.

Although some improvement can be obtained by using socalled grinding aids or by using air-swept mills in which ground material is carried out of the mill entrained in the air, there is room for further improvement, which it is our object in this invention to provide.

SUMMARY OF THE INVENTION Broadly stated the invention is an improved method of grinding material to a very fine size. The improvement is in initially grinding material to a fineness characterized by having a residue below l percent on a 2l0-micron screen, thereafter further grinding such material in the mill by means of grinding bodies having a maximum middle or average weight of less than grams until a product of a fineness characterized by having below 1 percent residue on a 2l0-micron screen is obtained and finally the product is passed out of the mill by gravity or overflow through an opening at the outlet end of the mill.

As is well known, the fineness of the product delivered by any grindingmill depends on the rate of feed, and (provided that the mill is of appropriate dimensions and rotated at an appropriate speed) reduction of the initially fine material to the very fine state can be ensured merely by ascertaining a suitable rate of feed and then maintaining this constant, either under manual control by the operator or automatically.

Preferably the mill is a tube mill with a hollow trunnion at the outlet end and (as described in US. application Ser. No. 647,152) has a built-in dam ring constituting a partition between a grinding chamber and a discharge chamber that leads to the trunnion, the discharge chamber being furnished with means for returning the grinding bodies to the grinding chamber through the dam ring. Thus the very small grinding bodies are continuously returned from the discharge chamber at substantially the same rate as they enter it, and at the most only a small proportion leaves the mill with the ground product.

So far as we are aware, such'very small grinding bodies have not previously been used in. a tube mill of the overflow type, but it has now been found that. the output capacity may be considerably increased without increasing the size of the mill by the means of the invention.

As the charge of small grinding bodies is capable of grinding a greater amount of material to a desired fineness, it is an important feature that the mill is constructed so that the material may flow in a continuous stream through it and out through an overflow opening in the end of the mill without being hindered in its movement by any diaphragm or having to be carried out of the mill by means of any airstream through the mill.

If the grinding bodies are extremely small and the production in the millis forced to a maximum, some grinding bodies may be forced out of the mill together with the ground product despite the provision of means for returning grinding bodies from the discharge chamber. Although the grinding bodies thus leaving the mill are an insignificant proportion of the total charge of grinding bodies in the mill, they should preferably be recovered and recycled to the mill, partly in order to save additional costs of a continuous minor replenishment of the charge of grinding bodies, and partly in order to keep the final product totally free from grinding bodies. When, as is usually the case, the grinding bodies are ferrous, the separation is advantageously effected magnetically.

The grinding according to the invention may be effected wet or dry. The invention is particularly applicable to the grinding of a washable material which is in a slurrylike state as a result of an addition of water. The small grinding bodies have a very suitable grinding effect on such material that is easily ground.

The invention is also applicable to a wet or dry material that has a minor constituent resistant to grinding. In such a case the grinding may be selective,.that is to say the material resistant to grinding passes substantially uncrushed through the mill and out of the mill together with the slurry. An example of such a material often present in the raw material used in the mixing of raw slurry for burning to cement clinker in a rotary kiln is a very hard flintstone, which is normally undesirable as a constituent in the ground slurry. This flintstone has nearly the same specific gravity as that of the basic material and cannot be separated off properly from the material before it is ground. In a conventional mill the major part of it is often ground completely together with the basic material, thereby making it impossible to separate the flintstone from the ground product after the grinding is finished. This is disadvantageous, because it is difficult to maintain the desired composition of the raw slurry. If, however, the flintstone passes through the mill unground, it may be separated from the ground product by screening and then separately ground, a suitable proportion of it being then added to the slurry. Thus it is possible to keep the amount of flintstone present in the raw slurry at a desired low level and to add a proportion to the slurry afterwards, should this be desired, because of the required analysis of the cement clinker.

in dry grinding the grinding bodies have less tendency to adhere to and to be carried away with the material than in wet grinding, so the grinding of dry material may be performed with the use of even smaller grinding bodies. Preferably, these have an average weight of less than 6 grams.

Although the mills used in the invention are preferably made as described and claimed in US. applicationSer. No. 647,152, the present invention includes as a novel plant the combination with the mill of a screening apparatus outside the mill for separating the ground product from material that has passed through-the mill without being ground, a magnetic separator for removing ferrous grinding bodies which have accidentally left the mill together with the ground material, and means for returning these grinding bodies to the mill.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation partly in section of one mill arrangement;

FIG. 2 is a side elevation partly in section of a second mill arrangement; and

FIG. 3 is a side elevation partly in section of a third mill arrangement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mill 11 shown in FIG. I has a grinding chamber 12 and hollow trunnions l4 and 18. The material is fed to the mill through a hopper 13 and the trunnion 14. The mill has a dam ring 16 with a central opening is surrounded by a frustoconical ring 20. On the outlet side of the dam ring 16 there is a discharge chamber 17 with a central discharge opening leading to the hollow trunnion 18. Around this opening there are scoops 19 for lifting the material and dropping it onto a screw 37 that has a short shaft of substantial diameter and a thread 36. This thread conveys material away from the mill and into the hollow trunnion 18 which itself is devoid of any thread. The end 21 of the screw 37 facing the opening in the dam ring 16 is frustoconical to facilitate the return of grinding bodies from the chamber 17 to the grinding chamber 12.

The grinding chamber 12 contains a charge of grinding bodies of an average weight less than l grams. Such grinding bodies would not be able to grind any very coarse material satisfactorily as their impact on it would have no appreciable effect. They can, however, very effectively grind fine material having a particle size distribution corresponding to a sieve residue below percent on a 2l0-micron screen. This is the material that is fed to the mill, and it may come from any suitable type of crushing or grinding machine preceding the mill 11.

The ground material passes through the central opening of the dam ring 16 to the discharge chamber 17 and thence out of the mill through the hollow trunnion. MOst of the grinding bodies are retained in the grinding chamber 12, as they have difficulty in passing through the opening 15 of the dam ring because of the provision of the ring 20. However, some of the grinding bodies do leave the grinding chamber and pass through central opening 15 to the discharge chamber with the ground product. A certain amount of grinding bodies will therefore be present in the discharge chamber at any given moment, but by the end 21 of the screw 37 grinding bodies are returned to the grinding chamber 12 through central opening 15 at a rate such as to establish an equilibrium between the grinding bodies leaving the grinding chamber and those returned to the grinding chamber.

The flow of ground material is not hindered by any screw inside the mill, and material may flow fast and freely through the opening 15 of the dam ring and out of the mill. The small grinding bodies could not be separated by means of conventional screening diaphragms as they would very soon block up the openings in these.

The mill shown in FIG. 2 has three discharge compartments beyond the main grinding compartment 12. These are shown at 22, 23, and 17', and the compartments 22 and 23 are separated by a dam ring 24, while the compartments 23 and 17 are separated by a dam ring 25. These dam rings are all identical with the dam ring 16 shown in FIG. 1. Improved separation of the grinding bodies from the ground material is obtained. In this mill the ground material is not forced out of the mill by means of scoops, but may flow freely through the hollow trunnion 18 after having passed through the discharge chambers. On the other hand the grinding bodies are as far as possible prevented from leaving the mill by a plate 38 which is suspended in the middle of the chamber 17' by means (not shown) and which carries a protrusion 39 of frustoconical shape. In addition the trunnion 18 has an internal thread 40 of such hand as to convey grinding bodies back into the mill.

In the plant shown in FIG. 3 the mill is essentially the same as that shown in FIG. 2, though there is no protrusion 39 on the plate 38. The trunnion 18 carries a drum sieve 27 having openings of about 2 mm. with an outlet channel 29, bounded in part by an end wall 30 of the sieve, through which the tailings, that is to say the residue which will not pass through the sieve pass. The channel 29 is also partly bounded by t e sieve (as described in U.S. application Ser. No. 680,502). The very fine ground material passes through the sieve into a hopper 28. The tailings pass into a hopper 31, passing a magnetic separator 32 on their way. This separator attracts any of the very small grinding bodies which may have passed into the sieve 27, carries them above a hopper 33 and there releases them. From the hopper 33 these grinding bodies are returned to the inlet end of the mill by a conveyor 34 and an elevator 35. The separator 32 also removes any other ferrous material that may be present.

The plant shown in FIG. 3 is particularly useful when flintstone or other constituent resistant to grinding is present. This constituent passes along the bottom of the sieve and leaves the drum sieve through the outlet channel 29, being discharged into the hopper 31, from which it is removed and treated separately.

It is a decided advantage that sorting of the grinding bodies, either to ensure that all those in a chamber are of the desired size or in order to discard grinding bodies that are too small, is not needed in the plant according to the invention.

Iclaim:

1. In a method of grinding material to a very fine grain size which includes,

grinding the material to a fineness in which the residue on a 2l0-micron screen is below l0 percent,

the improvement which comprises,

grinding the material in the grinding chamber of a mill employing grinding bodies having a maximum average weight of less than 15 grams to produce a ground product in which the residue on a 2 lO-micron screen is below 1 percent,

passing the resulting ground product and grinding bodies from the grinding chamber over a definite path into a discharge chamber,

returning grinding bodies delivered into said discharge chamber back along said path into the grinding chamber,

and

passing the thus ground product, the residue of which on a 2 l O-micron screen is below 1 percent, from the discharge chamber of the mill.

2. The method of grinding material in a mill according to claim 1 wherein the grinding bodies employed have a maximum average weight of less than 6 grams. 

2. The method of grinding material in a mill according to claim 1 wherein the grinding bodies employed have a maximum average weight of less than 6 grams. 